432 



HISTOLOGY 



cells in a transverse section of a pollen sac are larger and have propor- 

 tionally far larger nuclei and nucleoli than the peripheral cells. These 



latter cells become differen- 

 tiated as the nurse cells, 

 forming at a later stage 

 (Fig. 383, B) a double row 

 of cells whose nuclei divide, 

 by amitosis or sometimes 

 by a peculiar many-cen- 

 tered mitosis seen in the 

 figure, into two nuclei, 

 without a subsequent divi- 

 sion of the cytoplasmic 

 body. Some of them al- 



FIG. 384- -Pollen mother cell of Magnolia at period of read y have tw nudei in 

 completed growth and before any reduction processes Figure 383, A. 



have set in. x 1800. Meanwhile the central 



pollen mother cells have grown to the proportional size indicated in 

 Figure 383, B, where the outline of a full-sized pollen mother cell is 

 seen in contact with the double layer of nurse cells, now at their fullest 

 size and vigor. The double nuclei and the peculiar form of mitosis, 

 which is rarely seen among them, are well shown in this figure. The 

 ultimate fate of the nurse cells is shown in Figure 383, C, which rep- 

 resents the pollen sac at the time that each pollen mother cell has 

 divided by its two reduc- 

 tion divisions into four 

 young pollen cells. During 

 this time the nurse cells 

 have evidently been nour- 

 ishing the reproductive 

 cells, and now appear as 

 a thin layer of shrunken 

 cells, which are soon to 

 further disintegrate and 

 finally to disappear. 



A pollen mother cell, 

 such as is outlined in Fig- 

 ure 383, B, is better shown 

 by Figure 384, which shows 

 one at the maximum size 

 of its growth and while it 

 yet retains its primitive nuclear structure. The nucleolus is very large 

 and very perfect in outline and shows no vacuoles. The skein which 



FIG. 385. Pollen mother cell of Magnolia preparing 

 for reduction divisions, nucleolus vacuolated. Chro- 

 mosomes beginning to form. X 1800. 



